Leaf Topics rqt_graph

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As bvbdort mentioned, a leaf topic is a topic which has no subscribers. It is called that in reference to a tree structure which has branches and leaves. Elements without children are called "leaf nodes" or "end-nodes".

Since nothing is subscribing to a leaf topic, you usually are not interested in viewing it in the graph because its data is not in use. However, sometimes you need to know about a leaf topic, for example if it isn't suppose to be a leaf topic and you actually want something to be subscribed to that data!, so the option to display those topics can be toggled mainly for debugging purposes.

The problem you mention at the end of your question may be happening if you close rqt_graph, but do not close roscore, and then restart rqt_graph. There could be things still running that are showing up on the graph that you don't expect. To start fresh you can restart roscore when you restart rqt_graph. Or use the rosnode kill command to kill the nodes you don't want running. What I actually do is use rqt instead of rqt_graph, that way you can see the node graph and process monitor at the same time and it is easy to kill nodes you don't want.